Faraday Cage For Lightning Protection

[anorlunda]

A recent thread talked about a radio inside a metal box. That reminds me of a question I never resolved.

9 years ago my sailboat was struck by lightning. 6 people were on board, but nobody was hurt, luckily. My mast is grounded to an underwater plate via a thick copper cable inside a plastic conduit. But the EMP fried all my electronics including hand held devices. Since then, I try to protect my devices by putting them in the oven of my stainless steel stove, or in cookie tins. I have no proof of these protections being effective.

Lightning has many frequencies. My goal is not perfect shielding, but just enough attenuation of the EMP to let my devices survive. I expect that there is a lot of military research on this topic, but I don't have any references, or any way to compare weapons EMP with lightning EMP strengths.

Ovens and cookie tins, effective or old wives tales?

 

[berkeman]

A recent thread talked about a radio inside a metal box. That reminds me of a question I never resolved.

9 years ago my sailboat was struck by lightning. 6 people were on board, but nobody was hurt, luckily. My mast is grounded to an underwater plate via a thick copper cable inside a plastic conduit. But the EMP fried all my electronics including hand held devices. Since then, I try to protect my devices by putting them in the oven of my stainless steel stove, or in cookie tins. I have no proof of these protections being effective.

Lightning has many frequencies. My goal is not perfect shielding, but just enough attenuation of the EMP to let my devices survive. I expect that there is a lot of military research on this topic, but I don't have any references, or any way to compare weapons EMP with lightning EMP strengths.

Ovens and cookie tins, effective or old wives tales?

Do you have a microwave oven on-board?

 

[anorlunda]

Sorry, no microwave,

 

[PietKuip]

It is the magnetic field around the lightning bolt that is harming electronics. Its rapid rise induces voltages in current loops. Connected devices may have large loops, but also on a circuit board the induced voltages may be large enough to damage sensitive components.

 

[berkeman]

Sorry, no microwave,

There are a couple of things that you need to do to make an enclosure RF-tight against EM fields. I'll post some of those tomorrow (I'm at home now with a slow Internet connection).

 

[anorlunda]

Thanks Berkeman, I'll look forward to it. But are the RF frequencies the most damaging ones?

 

[berkeman]

I don't know much about the characteristics of EMP, so I don't know if the E or B field pulse and ringout does the most damage. But regardless, good RF shielding is the thing that is needed for keeping either out of a controlled volume. The biggest issue is sealing the joints/seams in the enclosure. For industrial RF shielding we use RF gasketing (spring finger strips) to help seal seams. I'm not sure yet what the easiest way is to replicate that cheaply in a home-brew enclosure...

 

[nsaspook]

Ovens and cookie tins, effective or old wives tales?

With the lid sealed with metal tape, the electronics disconnected and placed inside on non-conductive foam will reduce the chance of damage from a strike.
http://www.dtic.mil/dtic/tr/fulltext/u2/a234306.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870001225.pdf

 

[berkeman]

With the lid sealed with metal tape, the electronics disconnected and placed inside on non-conductive foam will reduce the chance of damage from a strike.
http://www.dtic.mil/dtic/tr/fulltext/u2/a234306.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870001225.pdf

What are the two fundamental properties of effective seal sealing? (according to Bell Labs)...

 

[berkeman]

LOL -- seal sealing...

 

[anorlunda]

Thank you napsaspook. You lived up to your handle. That is exactly the info my OP sought. (Plus bonus info on HEMP high altitude EMP)

Although a garbage can is hard to fit in the living space of a sailboat, the cookie tins might work if properly sealed. A quick search on Amazon shows that Duck brand aluminum tape is readily available.

I'll abandon my use of the oven. That would be nearly impossible to seal.

A regular size cookie tin can hold a couple of phones and a tablet, or a handheld VHF, or a small GPS chartplotter. Larger, or panel mounted equipment like VHF radio, SSB radio, radar and large GPS chartplotters, are too difficult to protect, But for coastal and inshore use, apps on my phone give me backups for GPS chartplotter, weather radar, and ship traffic AIS. My conclusion is that the small devices are the easiest and most critical ones to protect from lightning.

LED lights are highly vulnerable, and difficult to dismount to put inside a tin. I have oil lamps to back up cabin lights, and a weather proof oil lamp for use outside. To back up the red/green navigation lights, I use battery operated LED lights. I'll store those in a metal tape sealed tin. Batteries are removed from the devices and stored in the tins inside sandwich bags (in case of battery leakage).

There is also something called a Luci light. It is an inflatable LED LIGHT/solar panel. When deflated, it stores in the size and shape of a short stack of DVD discs. I'll put one of those in a tin.

Plastic packing korns in the tins provide the non-conductive foam bed NASA specified, and prevent rattles. Tins of various shapes and sizes are readily available online (and they come filled with delicious stuff :-)

Thanks everyone.

 

[anorlunda]

Whoops, I forgot a critical item. I also have a 12v solenoid propane shut-off valve installed near the propane tanks. That too may be fried by lightning. I have a spare solenoid that can be stored in a tin. But that may not help if the alternator, solar panels, solar panel, charge controller and even the main batteries are fried. I'll have to figure out a way to manually operate that solenoid plunger as a backup, so we can continue to cook without electricity.

Oh in case you ask, I do have contingency plans to plug holes in the hull that might be blasted out by lightning. My interest is not academic; it is survival in an inherently self-sufficient scenario. Next to getting run over by a ship,and ground up in the ships propeller (for which I have no contingency plan), lightning is the next most likely life-threatening scenario I face in blue water.

 

[berkeman]

For sealing up seams (like between your tin and the lid), you should be using some sort of EMI gasketing material like spring fingers. To make a good conductive seal, you need 2 things:

-1- Many points of contact

-2- Air-tight contacts

You don't get that generally with conductive tape, so it's best to use spring-finger gasketing. There are lots of styles, but in your case you would want to use some that has pointy contacts that face both ways. You stick the strip to the tin near the top, and then when you put on the lid, it compresses the pointy things and makes multiple contacts around the circumference of the tin. Obviously the tin and lid need to be unpainted and uncoated in the area where the contacts are being made.

http://hollandshielding.com/content/2400/2400-Twisted-Fingerstrip-main-image.png

 

[Jeff Rosenbury]

Lightning protection is tricky.

Some rules of thumb include using a direct grounding path and a well insulated Faraday cage which is grounded through an inductor and possibly a large resistor.

The most damaging part of lightning is the sudden surge of current. This current will often ionize air rather than follow a conductor around things. So keep your grounding path as straight as possible. Conversely, a looping path to ground with your Faraday cage will slow sudden current changes. (That assumes you need to ground your cage. Your millage may vary on this point.)

Amazon sells Faraday Bags.

 

[berkeman]

Amazon sells Faraday Bags.

Good idea -- put each thing in a Faraday bag before putting it in the Faraday cage/tin.

 

[meBigGuy]

Why isn't a small microwave, as a container, a good place to start (search microwave as faraday cage)? Seems really efficient size wise. Nice door, too. I don't know how good the shielding is, but that plus Faraday bags (what are new to me) seems like a good possibility. Also, you could give it its own ground.

 

[Baluncore]

The good conductivity of your lead to the keel connection results in a high current with a fast risetime and so a high magnetic field.

If you split the down lead into four identical parallel leads at the outside corners of the cabin then the fields inside the rectangle of conductors would tend to cancel. The cabin would then be a safer place to keep sensitive items.

 

[jim hardy]

Imagine the rod below is a lightning bolt.
Ten kiloamps is only a medium-large one and lightning's rise time is in microseconds.

While your Faraday cage stops an E-field, i think it needs to be conductive enough so that substantial Lenz's Law currents can flow in it opposing the B-field that encircles the lightning bolt.
That'll reduce magnetic induction inside the cage.
So while your oven may not be a perfect Faraday enclosure i think it'll help provided its metal case is thick enough to allow a lot of current flow.
I'm not sure whether a ferromagnetic skin would help further, but the galvanized iron garbage can suggested above seems intuitively "right"..

Thirty years ago i had to figure out why a lightning strike tripped our power plant. Induction , Biot Savart.

 

[Jeff Rosenbury]

The good conductivity of your lead to the keel connection results in a high current with a fast risetime and so a high magnetic field.

If you split the down lead into four identical parallel leads at the outside corners of the cabin then the fields inside the rectangle of conductors would tend to cancel. The cabin would then be a safer place to keep sensitive items.

I would advise against this. While it would work in theory, I suspect in practice most of the current would flow down one conductor. The return stroke would then flow up another creating a loop and amplifying the problem.

With some experimentation I think the system could be designed to work, but that sounds more like a research grant than practical lightning protection.

 

[Jeff Rosenbury]

Why isn't a small microwave, as a container, a good place to start (search microwave as faraday cage)? Seems really efficient size wise. Nice door, too. I don't know how good the shielding is, but that plus Faraday bags (what are new to me) seems like a good possibility. Also, you could give it its own ground.

I like the microwave idea. The size is convenient. Plus you get a microwave to cook diner. I suspect a Faraday bag in a microwave is plenty of protection. Microwave ovens are designed as Faraday cages. The frequencies of lightning are lower (~100kHz, ultra wide band), But I don't think that's a problem.

Still, if you have the extra room for the trash can on a small boat, use that. Trash cans are made of iron which isn't as good a conductor, OTOH the magnetic properties should channel the field around the interior. Both are good ideas.

 

[Finny]

You really should search something like Lightning protection in boats.

There are some good illustrative diagrams and pictures here:
http://www.bing.com/images/search?q...&qpvt=lightning+protection+in+boats&FORM=IGRE

Also, see the "PRINCIPLES" section here:
http://www.marinelightning.com/

The American Boat and Yacht Council [ABYC] has recommendations here:
http://www.kp44.org/LightningProtectionABYC_Standards.php

A significant issue with boats are side discharges of a lightning strike which may travel to thru hulls and blow them apart as well blow holes through a hull near the waterline. As near a vertical lightning ground as possible is desireable. Keep lightning ground separate from bonding [often bare wire] and ac and dc ground wiring.

One way to help lightning find a safe path is to clamp a #4AWG or larger wire to each mast stay with an underwater conductor at the other end. Turns out a serrated underwater conductor edge with maximum edge length seems to help disippate lightning most effectively. So multiple strips may work better than, say, a square conductor of equal area.

I used to hang a big commercial zinc overboard and connect via conductor to wing nut to a lighting rod when dockside. That commercial zinc weight keeps the lightning ground underwater in high thunderstorm winds.

 

[tech99]

You really should search something like Lightning protection in boats.

There are some good illustrative diagrams and pictures here:
http://www.bing.com/images/search?q...&qpvt=lightning+protection+in+boats&FORM=IGRE

Also, see the "PRINCIPLES" section here:
http://www.marinelightning.com/

The American Boat and Yacht Council [ABYC] has recommendations here:
http://www.kp44.org/LightningProtectionABYC_Standards.php

A significant issue with boats are side discharges of a lightning strike which may travel to thru hulls and blow them apart as well blow holes through a hull near the waterline. As near a vertical lightning ground as possible is desireable. Keep lightning ground separate from bonding [often bare wire] and ac and dc ground wiring.

One way to help lightning find a safe path is to clamp a #4AWG or larger wire to each mast stay with an underwater conductor at the other end. Turns out a serrated underwater conductor edge with maximum edge length seems to help disippate lightning most effectively. So multiple strips may work better than, say, a square conductor of equal area.

I used to hang a big commercial zinc overboard and connect via conductor to wing nut to a lighting rod when dockside. That commercial zinc weight keeps the lightning ground underwater in high thunderstorm winds.

I think the most important action to take is to remove all cables from the item. Unfortunately, I believe that even a compass can be upset by a strike, but sailing boats can of course be sailed with no electronics at all - as we did for the first few millenia - and it is a much more interesting experience.

 

[Baluncore]

While it would work in theory, I suspect in practice most of the current would flow down one conductor. The return stroke would then flow up another creating a loop and amplifying the problem.

You suspect wrongly. Fast risetime pulses are shared by the inductance of the multiple paths available. The important thing is to use external uninsulated conductors, the ground connections should be outside the cabin as external surface currents will travel faster than internal cabin currents or currents on insulated cable. The four external bare conductors form a lower inductance cage than a single internal conductor to the keel which will have the highest possible inductance and lowest velocity.

Lightning strike currents usually jump away from the hull about 20” or 500mm above the water line. That is because the external air path has lower inductance than the hull surface path through the dielectric water. The internal path down a single wire to the keel is actually the most unlikely path.

If a return stroke flows then it occurs about one millisecond after the first strike. That cannot make a loop because the strike and return currents are flowing at different times. The magnetic fields of the strike and the return will be about 300 km apart. But they will individually cancel with themselves inside a 4 conductor cage.

 

[anorlunda]

The microwave sounds great but it is very big and bulky. I couldn't power it anyhow. I have only 12 vDC and max load on board is 60w.

The ESD pouches sold on Amazon claim 45 db EMI attenuation 1-10 Ghz. I have no way to judge that number. Do I need 5 db, 50 db, 500 db, or what for lightning protection? The paper nsaspook cited shows that the rise time of the spike is 0.3 microseconds.

By the way, my internet research on this topic led me into a morass. EMI shielding, EMD static discharge, EMP survival in case of nuclear war, surge protectors for AC wired devices, survivalists and tinfoil underwear. None of all that was as relevant or to the point as that advice I've gotten here on PF. Thank you all.

I'm going to go with ESD pouches inside tins sealed with metal tape, for two reasons. I priced the RF gaskets at $60/m (too high.) A 10 gallon garbage can might be good, but I want to store my protected devices in my ditch kit (a canvas bag full of survival equipment that I grab before jumping into the life raft I.C.E.) The tins are a more practical size.

By the way, you may also be interested to hear that even grounding the mast is controversial in boating circles. Some argue that extending the ground 50 feet up in the air when on open water makes the boat more likely to be hit in the first place. It is my belief that grounding the mast helps prevent blasting a hole in the hull, but it does nothing for EMP damage. The other risk comes from a vaporizing ground cable spraying occupants with molten copper. I enclose the cable in plastic electrical conduit, hoping that is enough.

EMP damage can occur with a hit on the grounded mast, or a hit on the deck with ungrounded mast, or a hit in the water, or even a hit on a nearby boat. In the past case when my electronics were fried, the main hit was on another boat 50 feet away. No wonder the subject is complicated.

 

[jim hardy]

In the past case when my electronics were fried, the main hit was on another boat 50 feet away.

That rather supports Piet's statement in post #4.

My one maritime experience was a non-event, but gave me reason to think... A storm overtook us in the ocean a few miles out.

I believe this about lightning, from an old Westinghouse "Transmission and Distribution" handbook...

Lightning begins up in the air at altitude where it's easier to ionize the rarified air molecules.
It tracks downward in fractal looking fingers at only milliamps until it reaches earth. That track is so faint as to be usually not visible in daylight.
When one of the fingers reaches the ground , current increases to hundreds or thousands of amps with microsecond risetime. The air path being already ionized is almost a short circuit .
That intense current makes the explosive "Flash" we see, and that flash progresses from the ground up because energy deposition is greater in densest air near ground.

We felt the electric field. It was making our hair stand on end and the rigging "hissed".
This was a 23 foot daysailer not a serious blue water boat, it had no lightning protection at all.
Thinking about being at the bottom of lightning bolt ,,,,
I placed wires from the rear stay and both side stays overboard. My thinking was this -
should one of those electric fingers find my aluminum mast i wanted the explosive current to vaporize my wires and create an already ionized path outside the hull not through the mast step and fixed keel.. That'd place the explosion outside the hull where i'd prefer to have it. Hopefully the voltage drop across that already ionized path would not be enough to flash through the hull.

Moving "Ground" up to the top of the mast let me invite lightning to take a non destructive path to the surface.

Luckily we were not struck, only saw lightning well behind us. Surfed right over that sandbar at Bimini harbor on a big roller in 60 mph wind.Were i building a seagoing yacht i'd provide for lightning wires from the standing rigging that i could deploy to surround the vessel.with a 'preferred path envelope' .
Much as Finny described in post #21.

Seem sensible?.

 

[meBigGuy]

There are some pretty small microwaves out there. Westinghouse WCM660B 10.3 x 17.8 x 13.6 inches ; 23.1 pounds
Maybe that isn't small by "room on a boat" standards.

Maybe you could use a safe?

 

[Jeff Rosenbury]

The ESD pouches sold on Amazon claim 45 db EMI attenuation 1-10 Ghz. I have no way to judge that number. Do I need 5 db, 50 db, 500 db, or what for lightning protection? The paper nsaspook cited shows that the rise time of the spike is 0.3 microseconds.

I sympathize. I suspect 45dB is plenty. I would have two worries. The first is the frequency. The frequencies you are worried about are much lower. While most Faraday cages work better at lower frequencies, I would still be concerned. My second concern is the quality of that figure. While looking for an antenna once at work, we tested 20 of them we bought off the internet. (We do this for a living, so they all looked reasonable.) Only one of them worked anywhere near the promised VSWR of 3:1.

I would consider one bag inside another from another company.

If you are really dedicated, you can test them with an AM radio and a car. Place the radio in the bag tuned to a low frequency station. Listen. Drive closer to the station's antenna. (Remember the cat is a good Faraday cage itself, so you'll need to get out periodically to check reception.) See how close you need to get to overpower the bag. If the radio is of high quality you can find out its minimum signal strength (often about 90 dBi as I recall, but it varies a lot.) You can find the station's power output as well (by calling them if you can't find it on the internet.) From that you can calculate the actual loss through the bag. Hopefully the bag works so well at lower frequencies you will need to get very close to the AM antenna. If so, you're good.

 

[Jeff Rosenbury]

You suspect wrongly. Fast risetime pulses are shared by the inductance of the multiple paths available. The important thing is to use external uninsulated conductors, the ground connections should be outside the cabin as external surface currents will travel faster than internal cabin currents or currents on insulated cable. The four external bare conductors form a lower inductance cage than a single internal conductor to the keel which will have the highest possible inductance and lowest velocity.

Lightning strike currents usually jump away from the hull about 20” or 500mm above the water line. That is because the external air path has lower inductance than the hull surface path through the dielectric water. The internal path down a single wire to the keel is actually the most unlikely path.

If a return stroke flows then it occurs about one millisecond after the first strike. That cannot make a loop because the strike and return currents are flowing at different times. The magnetic fields of the strike and the return will be about 300 km apart. But they will individually cancel with themselves inside a 4 conductor cage.

Do you know anyone who has done this? Did it work?

While I agree with your basic model and I think it would work on a boat centered under the storm in a glass calm, those don't typically go together.

What if there's more potential on one side of the boat than the other? (The potential at sea should theoretically mirror the one in the clouds.)

What if water gets on or near one conductor acting as a dielectric?

What if a hundred other things?

Models don't always match reality. Experience should guide us as well. I would follow the recommendations of the boating folks Finny linked to. They seem to know what they are talking about. They also recommend multiple paths to water, but they do so in order that one of them will work, not to distribute the load.

I found it interesting that one of them recommended a sort of reverse lightning rod for catamarans so the path led straight through the bridge between the hulls and jumped to the water. But they also recommend lots of connections to all the metal parts of the boat.

 

[anorlunda]

Above (to the best of my primitive artistic abilities) is my pre-exising grouding system. Grey is hull, blue is water, and green is conductive metal with a path to the grounding plate. The mast (49' elevation) is grounded to a 2 square foot plate at elevation -2'. The side stays and external chain plates lead from the mast head down to within 1 foot elevation port and starbord, 0 feet forward, and 3 feet aft. I think that is a fair approximation of splitting the conductors 4-ways to make a cone of protection as suggested by Baluncore and Finny.

The ABYC lightning protection standard that Finny linked is (pardon me) B.S. ABYC goes off the deep end sometimes. The standard says:

"All large metal objects above and below decks should also be electrically tied into the lightning ground conductor. This is a precaution against side flashes. Large metal objects include shrouds, chainplates, toe rails, sail tracks, winches, steering wheels, and bow and stern pulpits. These items can be tied into the ground conductor wire by a minimum #8AWG stranded copper gauge wire, or connected directly to the hull ground terminus."

​

I count more than 36 such large metal objects on my boat (48 if I include every pot and pan in the galley). If each had its own #8 grounding wire, the boat would look like a spider web. I've seen hundreds of cruising boats, including those freshly delivered from the factory to ABYC specs. Not a single one could be said to comply with that standard. I know that most of you live in houses rather than on boats. Many of the same lightning protection ideas can be applied to a wood frame house. Try to imagine applying ABYC's large metal object standard to your own house.

I am reminded of the deer whistle I had on the bumper of my car years ago, that claimed to make a tone above the range of human hearing. I never hit a deer before having the whistle, nor after. Did the whistle work? If I did hit a deer with the whistle on, does it prove that the whistle doesn't work? How would I know the difference between a functioning and non-functioning whistle? Some questions can never be answered anecdotally only satistically and with adequate controls.

But we (myself included) are getting off topic. The OP asked not about protection for the boat, but for protecting devices like cell phones and tablets carried on board.

But we (myself included) are getting off topic. The OP asked not about protection for the boat, but for protecting devices like cell phones and tablets carried on board.

 

[Baluncore]

There is no electric charge on the inside of a conductive cage because like charges repel, nor are there strong magnetic fields inside because external currents cancel. Aircraft, cars and their contents survive strikes every day without problems. That is because, like the cabin and pyramidal rigging of a boat, they have an external cage of conductors.

Do you know anyone who has done this? Did it work?

Exactly what do you mean by "this?"? I have designed, and built professional antenna systems that model much like yacht rigging and which never fail while surviving lightning strikes often.

What if there's more potential on one side of the boat than the other?

How is a differential potential possible? The tip of the mast and the sea surface can only have one potential difference.

What if water gets on or near one conductor acting as a dielectric?

The strike does not travel in the conductor but on or above the surface, It travels as a wavefront over the conductive structure. That surface breakdown current is guided by the conductor potential but travels outside or away from insulation on the conductor. Breakdown paths through gasses are curved = arc. Conducted currents avoid sharp bends and dielectric insulation.

What if a hundred other things?

Now that you have resorted to credibility assassination by FUD, (Fear, Uncertainty and Doubt), you have lost credibility.